Thus, FR 2 795 360 recommends using a mold having two undercut-forming portions and one base portion. For unmolding, firstly the two undercut-forming portions are moved apart, and the thermoformed object is held against the base portion of the mold by suction, said base portion being provided with suction ducts. Said base portion can then be moved to extract the object from the zone in which the undercut-forming portions of the mold are situated, in order to make it accessible by pick-up means that make it possible to transfer said object to the outlet of the device.
That known device is relatively complicated because it implements holding the object by suction against the base of the mold. The effectiveness of that suction must be excellent in order to enable the object to be held correctly while the base portion of the mold is being moved.
In particular, in order to avoid the risks of the object tipping over relative to said base portion, it is necessary for said base portion to cover the base portion of the object over a height that is sufficient. Once the base portion of the mold that is carrying the object has been moved far enough away from the undercut-forming portions of the mold, it is necessary to cause the suction to cease, and to separate the object relative to said base portion. This requires the object and the base portion of the mold to be moved relative to each other parallel to the direction in which the thermoforming piston is moved, in order to extract the base portion of the object from the base portion of the mold, and then requires the object to be transferred sideways by moving it away from the thermoforming tool.
Thus, the way the various parts move relative to one another is relatively complex, and, if it is assumed that the direction in which the thermoforming piston is displaced is vertical, the unmolding operation firstly requires the undercut-forming portions of the mold to be moved horizontally so that they move away from the undercut portion of the object, it then requires the base portion of the mold to be moved vertically over a first stroke sufficient to move the rim of the object away from the undercut-forming portions of the mold, and said base portion to be moved vertically or else the object to be moved vertically relative to said base station over a second stroke sufficient to unmold the base portion of the object relative to the base portion of the mold, and finally, it requires the object to be transferred horizontally. In addition, in general, the entire mold (including the undercut-forming portions must be capable of being moved over a small vertical stroke in order to clamp the thermoplastic material between the mold an the countermold.
FR 1 427 784 discloses another device in which, for unmolding an object presenting an undercut portion, firstly the undercut-forming portions of the mold are moved apart, and then the object is expelled from the base portion of the mold by an ejector piston whose head forms the base of the mold during thermoforming. During that expulsion, the object is merely carried by the head of the piston without being held relative to it, and there exists a risk that it might tip over or fall, thereby giving rise to wastage, or at least to reductions in manufacturing throughputs. In addition, the device of FR 1 427 784 is possible only when the mold and the countermold constitute respectively the bottom tool and the top tool of the thermoforming device. In other words, the object must be thermoformed downwards.
FR 2 256 818 discloses yet another device in which, after thermoforming an object from a strip of thermoplastic, a front portion of the mold, which portion closes said mold on the downstream side in the direction of advance of the strip, is moved away so that the strip can advance through one step by extracting the object that has just been thermoformed from the mold via the opening formed due to the front of the mold being opened. That device can be used only for thermoforming objects from a continuous strip of thermoplastic material.